Theoretical analysis of the variation of shear acoustic wave velocity in a piezo layer by adjoining viscous liquids

This paper presents the theoretical formulas for representing the variation of shear acoustic wave velocity in a piezo layer substrate as the function of the physical properties of adjoining liquids including the viscosity, μ, and dielectric constant ε_l, of liquids. The various configurations of sensors are discussed; in most cases of structures for sensing the viscosity of liquid μ, the sensing surface of piezo layer must be metallized for ignoring the effect of dielectric constant of adjoining liquid, the expression for the cases of the substrate with high piezoelectric constant (PZT-4, ZnO etc.) and normal thickness (for example, 0.5 mm at 20 MHz operation frequency) are derived. The structures without metallized surface for detecting the liquid dielectric constant are also discussed. The results show that the variation of shear wave velocity is very sensitive to the relative dielectric constant, Rε, which is defined as a ratio of dielectric constant ε₁₁ of substrate and dielectric constant, ε_l, of liquid